Chain-transfer constants, various monomers

Table 1. Polymerization and Chain-Transfer Constants for Various Monomers ...

Using the methods described, the values of Cm and Ci in the benzoyl peroxide polymerization of styrene have been found to be 0.00006 and 0.055 respectively [Mayo et al., 1951]. The amount of chain transfer to monomer that occurs is negligible in this polymerization. The chain-transfer constant for benzoyl peroxide is appreciable, and chain transfer with initiator becomes increasingly important as the initiator concentration increases. These effects are shown in Fig. 3-7, where the contributions of the various sources of chain ends are indicated. The topmost plot shows the total number of polymer molecules per 105 styrene monomer units. The difference between successive plots gives the number of polymer molecules terminated by normal coupling termination, transfer to benzoyl peroxide, and transfer to styrene. 

Equation (6.148), often referred to as the Mayo equation, shows the quantitative effect of various transfer reactions on the number average degree of polymerization. Note that the chain transfer constants, being ratios of the respective rate constants for chain transfer (Rtr) to the rate constant for propagation (kp), are dimensionless quantities dependent on the types of both the monomer and the material causing chain transfer as well as on the temperature of reaction. 

Fig. 14A-C. Isobutylene polymerization by TMPCl/TiCl4 and HX /TiCl4 at — 60 °C A,B determination of chain transfer constants in systems with slow initiation plus chain transfer to monomer C N vs conversion plot at various ED concentrations in the presence of initiator ([TEA] = (mol/l) =5-10 A = 1.5-HT3, = 3-l(T3, = 6-103) and in the absence of initiator (O, A, , V)...

Chain-Transfsr Agents. Radical chain-transfer agents, eg, various thiols, have been applied over several decades to control the molecular weight of the polymerization or to terminate the growing polymer chain, eg, in the case of telomerization (38). Recently, we have observed that the chain-transfer constants of the water-soluble A(-acetyl-L-cystein to RAMEB-complexed monomers, eg, methyl methacrylate and styrene, are three times higher in water than in the case of a mixture ofiV,A( -dimethylformamide and water. 

The chain transfer constants of various monomers at 60°C are presented in Table 3. The monomer chain transfer constants Cm are generally small 10 -10 " ) for most monomers because the reaction involves breaking the strong vinyl C-H bond ... 

This equation gives the fundamental correlation of the number average degree of polymerization with the rate of polymerization and the various chain transfer constants. Performing a polymerization experiment with only low conversion of monomer to polymer, the concentration of polymer is often too low to show significant chain transfer. The same holds true for the initiator, which is mainly used in the range of low concentrations. Without addition of solvent and additional chain transfer agent, eqn (1.55) reads after introduction of eqn (1.17)... 

Throughout this section we have used mostly p and u to describe the distribution of molecular weights. It should be remembered that these quantities are defined in terms of various concentrations and therefore change as the reactions proceed. Accordingly, the results presented here are most simply applied at the start of the polymerization reaction when the initial concentrations of monomer and initiator can be used to evaluate p or u. The termination constants are known to decrease with the extent of conversion of monomer to polymer, and this effect also complicates the picture at high conversions. Note, also, that chain transfer has been excluded from consideration in this section, as elsewhere in the chapter. We shall consider chain transfer reactions in the next section. 

A plot of the experimental data as the left side of Eq. 3-110 versus Rp yields a straight line whose slope is (C kt/ /Trf M 3). The initiator transfer constant can be determined from the slope because the various other quantities are known or can be related to known quantities through Eq. 3-32. When chain transfer to monomer is negligible, one can rearrange Eq. 3-109b to yield... 

In THF, however, no difference in the monomer reactivity ratios was observed between the (S)-MBMA-TrMA and (RS)-MBMA-TrMA systems, and the ratios (r =0.39 and 2- . ) showed similar reactivity of MBMA (Mi) and TrMA ( 2). The copolymerization seemed to proceed without termination and chain transfer reactions. An abnormal optical property was observed in some of the copolymers of (S)-MBMA and TrMA. Table shows the tacticity and optical data of the copolymers which were obtained in various polymer yields from the monomer mixtures of a constant molar ratio, [Mllo/[M2]o = The (S)-MBMA content in the copolymers decreased... 

Cessation of the growth of PVC radicals is caused almost completely by chain transfer to monomer (Section 6.8.2) rather than by termination by disproportionation or combination. In other words, the relative magnitudes of the various terms in Eq. (6-75) are such that the controlling factor is the CM(=k,r.M/ p) term. Since the ratio of these rale constants depends on tem Terature, the number average molecular weight of the product polymer is controlled simply by the reaction temperature and shows little dependence on initiator concentration or rate of polymerization. 

Problem 6.32 Nonideal behavior in polymerization of a given monomer is reflected in the variability of / [I] [M] at a constant temperature. Derive an equation for i / [I] [M] representing in the most general form the perturbations introduced by primary radical termination and various chain transfer reactions. Neglect, however, the recombination reactions of primary radicals and of radicals formed as a result of chain transfer, as also cross-combination of these types of radicals, all of which are likely to be insignificant except under unusual conditions. 

Table 20-8. Transfer Constants Cr as a Function of Chain Length in the Telomerization of Various Monomers with CCI4...

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